It is getting incredibly difficult to obtain a non-backdoored smartphone nowadays.

I tried to find which phones support alternative OSes, without Google control and telemetry, but it turned out that alternative OSes (LineageOS, PostmarketOS, Graphenos) support mostly support outdated models and it makes no sense to buy them. There is also "Google Pixel", but the prices start at around $600 which is 3 times more than a reasonable price for a phone.

So now I am wondering if it is possible to extract the ROM from a reasonably priced Samsung phone, remove the components I don't like and write it back.

GrapheneOS supports the newest Pixels, and only the Pixels that are still getting updates from Google. Right now the least bad option is probably a one-generation-old NOS or Open Box Pixel with GrapheneOS.